Jack Stall

[Chesty Morgan]

...I think you call it.

As far as I can recall (being a mere fixed winger), in a helicopter, you can reach an airspeed at which your rearward moving blade doesn't have enough speed to generate lift. Although I stand to be corrected!

What happens next then? Is it controllable?

Cheers

Chesty

[tomstheword]

I think youll find it is called Retreating Blade Stall. the aircraft will pitch up and then roll left or right depending on which way the blades spin.

[Flying Lawyer]

What you’ve described is Retreating Blade Stall

Just as the stall of an aeroplane wing limits its low speed, the stall of a rotor blade limits the high-speed potential of a helicopter.

The airspeed of the retreating blade slows down as forward airspeed increases. It has to produce an equal amount of lift to that of the advancing blade so, as the airspeed of the retreating blade is decreased with forward airspeed, the angle of attack of that blade must be increased to equalise lift throughout the rotor disc area.
If the angle is increased further and further to equalise the lift, the retreating blade will eventually stall - at some high forward airspeed.

First indication is often a vibration - the equivalent of buffet in a fixed-wing.
The nose lifts and there's a tendency for the helicopter to roll in the direction of the stalled retreating blade.
If the pilot didn’t reduce the pitch of the blades, the stall would become worse, vibration would increases and control might eventually be lost.


Is it controllable?
Yes, in the same way that you don't allow a fixed-wing to stall - by recognising the warning signs and acting accordingly.
Just as in a fixed-wing you take action at the buffet stage to prevent an incipient stall developing into a full stall, so you take action in a helicopter.
In both cases, you reduce pitch.

High forward speed is only one of the circumstances in which RBS can occur; there are other causes.


NB: There are hundreds of expert pilots on this forum - I'm not one of them.
This is just a 'broad brush' answer until one of them explains it more accurately.


FL

[MightyGem]

Jack stall is associated with the hydraulic servos/jacks that operate the main rotor blades. In certain helicopter(AS350/355, Gazelle are ones that I know of), during hard manouvering, the aerodynamic loads on the blades can become high enough to prevent the hydraulics from being able to move the blades further.

In the Gazelle, there is an uncommanded pitch up and roll to the right. Simply reducing the severity of the manouver will reduce the load and free the controls.

[Chesty Morgan]

Cheers guys, just what I was after:ok:

Is it wrong to assume that when the nose pitches up, as a result of RBS, you actually gain height?

FL I'm just trying to get my head around the physics of your spinning bits. When you say you reduce pitch to prevent, or recover, from RBS, I take it that this is the pitch of the blade and has the effect of reducing the AoA of the stalled blade. I understand that bit as that's what we do on fixed wings.

The thing that's confusing me - When we stall it, usually, happens to both wings simultaneously. Assuming you had entered a fully developed stall, as I picture it, when you carry out your stall recovery why does it not aggravate the symptoms of the stall and increase the roll rate or AoB?

Chesty (Confusing myself) Morgan

[NickLappos]

The classis signs of RTS are the loss of cyclic control, usually a gradual thing (because RTS is a gradual thing) where the lateral cyclic trim (where the cyclic needs to be to keep the wings level) begins to move rapidly toward the retreating side. This is accompanied by some increase in n/rev vibration, usually. Eventually, the cyclic hits the stops and the aircraft rolls toward the advancing side. At the same time, the nose tries to lift, so the cyclic needs to go somewhat forward. The reason why the aircraft behaves assymetrically, as compared to airplanes, is that the rotor is losing lift on one side only.

[Flingwing207]

<<trim & snip>>
...where the lateral cyclic trim (where the cyclic needs to be to keep the wings level) begins to move rapidly toward the retreating side. Eventually, the cyclic hits the stops and the aircraft rolls toward the advancing side. At the same time, the nose tries to lift, so the cyclic needs to go somewhat forward. Nick, am I way off base, or did you reverse the bolded bits?:confused:

[212man]

Too much time in a G550; the only retreating bit on those is the hostie retreating to the galley to fetch a G and T!!!;)

[B Sousa]

Used to hear that a lot of folks have met Jack Stall while flying in the Grand Canyon on super windy days. Never met the guy myself but the stories are enough to make your ass pucker.

[eagle 86]

According to my recollection and my RAF CFS notes the helicopter rolls towards the retreating blade and pitches up. The retreating blade commences to flap down as it passes through the 12 o'clock and reaches a maximum rate of flap down at 9 or 3 o'clock depending upon direction of rotation reaching its lowest point at 6 o'clock.
GAGS
E86

[NickLappos]

Reversed bits, sorry guys. Hasty response! Didn't re-read my response, had to run to a Gulfstream and fly around the world....

The cyclic drifts toward the advancing side, with a pitch-up! The effect is actually gradual if you approach the stall via low g and high airspeed. It can be very sharp if you experience it via rapid cyclic pulls to create higher g.

[Bravo73]

I'm just trying to get my head around the physics of your spinning bits.

Chesty,

Get hold of a book called 'Principles of Helicopter Flight' by a guy called W.J.Wagtendonk.

His book will probably answer all of your questions concerning the physics of our 'spinning bits'. His is certainly the volume that 'we' turn to in order to answer the questions ourselves. :ok:


HTH,

B73

[Heliport]

B73

Good book, but it's hardly worth Chesty going to the expense of buying it.

There's enough expertise on this forum to answer his question.

[HillerBee]

Correction on the fixed wing side, one blades usually stalls a fraction earlier and the airplane 'drops' a wing, if not recognised you'll enter a spin.

On the rotary side: it's no rule that there's is buffeting, it can just pitch up and roll without any warning. However as menitioned before the only recovery is to lower collective and reduce speed.

[Bravo73]

There's enough expertise on this forum to answer his question.

Indeed, H, but (as you're well aware), you can often get conflictory or confusing opinions from different self-appointed 'experts' on this forum. It takes time to work out which experts know what they are talking about and which 'experts' don't.

Better to go straight to the source, I reckon. FleaBay might turn up cheaper copies (if 20 quid is too steep for Chesty).

[topendtorque]

I'm just trying to get my head around the physics of your spinning bits.

When you do let us know about it sport.
Ask Bell why the blades flap, get a million answers, most anyone can tell you how to make them do it and what happens - when etc. but the physics, fluid hydraulics etc. - ???

Another book from the archives; 'The helicopter and how it flys' by John Fay, an Englishman - lent my copy eons ago - was not returned. Read it twice and passed the OZ theory easy.

[5711N0205W]

For books try a site called abebooks.co.uk (an online second hand book seller) - I did a quick search there and found both there easily.

[Pandalet]

The second-hand part of amazon is also pretty decent - find the book you want, then click on the 'used and new' link.

[Chesty Morgan]

Thanks guys.

[B Sousa]

Excuse me for not being an engineer, but wasnt the subject about Jack Stall, not Retreating Blade Stall?? If I recall Jack Stall was something found in the AS350 after lots of cyclic movement and may commonly be known as a Hydraulic Hiccup or Cyclic Lock. Leave it alone for a few seconds untill hydrualics get back to normal??.
OK, now make me look stupid..........

[Heliport]

Bert

Although Chesty said "Jack stall ..... I think you call it" he asked about a helicopter reaching "an airspeed at which your rearward moving blade doesn't have enough speed to generate lift."


B73
We're all entitled to our views. Mine is that there numerous people here who can properly be called experts. Some are world-renowned test pilots, others are less well-known but also experts. The amount of expertise and experience never ceases to amaze me. Thankfully, we don't have many "self-appointed 'experts' on this forum." Even genuine experts disagree occasionally.

[Rich Lee]

Excuse me for not being an engineer, but wasnt the subject about Jack Stall, not Retreating Blade Stall?? We are used to thinking of aerodynamics as individual elements because it is usually more simple to understand them that way. The truth is that these individual elements interact and create behaviours that do not always manifest themselves as a single aerodynamic problem.

Retreating blade stall is not a simple problem of exceeding Vne. AOB, pitch/roll rate, main rotor RPM, atmospheric turbulence and other factors can cause RBS well below Vne. The classical indications of RBS are as Nick described (as ammended). The first indications are an increasing main rotor vibration at main rotor n/rev followed by increasing rates of longitudinal pitch-up acceleration and then finally the lateral roll tendencies of deep RBS. Hydraulic, SAS and some Autopilot systems can mask incipient retreating blade stall indications. Pilot control technique can aggravate or mitigate the onset of RBS.

Consider jack stall and retreating blade stall. Either one can be the cause of the other. Jack stall can cause the aircraft to pitch or roll in a way that causes retreating blade stall. The pitch up tendencies of RBS can cause jack stall.

Often RBS can be confused with main rotor tip mach, a more nasty problem in my opinion because there are few indications it is about to happen, and when it happens the rotor blade response becomes rather unpredictable.

Few books discuss RBS in a holistic way, most consider it as an individual problem and this might be what Heliport is alluding to - here the discussion usually covers all the salient points of interest.

[Heliport]

Rich

That is what I was alluding to. I think most of us learn much more from rounded/broader discussions here than we ever did when studying individual topics for test purposes.
I also firmly believe that the combined expertise of the members of this forum is the finest single source of information available on helicopter topics.

[Nigel Osborn]

Many years ago when I was a young sprog pilot with a mere 300 hours, I was number 5 pilot in a line astern formation in a S55. The lead pilot was flying near vne & made a 30 or more degree bank turn to the right. I blindly followed at No 5 & probably was past vne & at least 40 degrees of bank. A few seconds later I was about 50 degrees nose up & 60 degrees of bank in a left turn & increasing. Needless to say the sphincter muscles contracted. The S55 being a grand old lady, seemed to sort herself out & no harm was done but I made sure I didn't do that again.
So I can't recommend trying to get RBS or similar.

[bryan_cfi]

Hydraulic Control System Servo Transparency Phenomenon

On December 19, 2003, the FAA issued Special Airworthiness Information Bulletin (SAIB) No. SW-04-35 alerting owners and operators of Eurocopter France AS350B, BA, B1, B2, B3, D, AS355E, and EC120B model helicopters that pilots can encounter a phenomenon known asservo transparency, servo reversibility, or jack stall during high airspeed, high collective pitch, high gross weight, high G-loads, and high density altitude conditions. The bulletin stated, in part, the following:
Pilots and operators may misunderstand this phenomenon. This aircraft phenomenon occurs smoothly, and can be managed properly if the pilot anticipates it during an abrupt or high load maneuver such as a high positive g‑turn or pull-up.
Servo transparency begins when the aerodynamic forces exceed the hydraulic forces and is then transmitted back to the pilot’s cyclic and collective controls. On clockwise turning main rotor systems, the right servo receives the highest load when maneuvering, so servo transparency results in uncommanded right and aft cyclic motion accompanied by down collective movement. The pilot control forces to counter this aerodynamically induced phenomena are relatively high and could give an unaware pilot the impression that the controls are jammed. If the pilot does not reduce the maneuver, the aircraft will roll right and pitch-up.
You should understand that servo transparency is a natural phenomenon for any flyable helicopter. BASIC AIRMANSHIP should prevent encountering this phenomenon by avoiding combinations of high speed, high gross weight, high‑density altitude, and aggressive maneuvers, which exceed the aircraft’s approved flight limitations.
During the investigation of another accident 29 (http://www.ntsb.gov/publictn/2007/AAB0703.htm#29) in which the servo transparency phenomenon test data were reviewed, Eurocopter representatives stated that the transparency phenomenon is transitory (lasting only 2 to 3 seconds) and can be immediately corrected by pilot actions to reduce G loads. If uncorrected by the pilot, the resultant aircraft reaction to the servo transparency will reduce the factors that contribute to the severity of the maneuver. The Eurocopter representatives also stated that the controls are fully operable throughout the entire transparency event; however, the force required to effect movement of the flight controls against the rotor system dynamic feedback loads increases significantly. Eurocopter stated that the force feedback for each control channel would be dependent in part on the amount of G loads experienced; however, the company estimated that about 22 pounds of force would be required to move the collective in the “up” or increased-pitch direction, with the same amount of force required to move the cyclic to the left. In addition, flight test data indicated that servo transparency could not be encountered if the collective was raised less than 50 percent.
Eurocopter’s chief pilot and an FAA test pilot conducted additional servo transparency test flights in December 2003. Flight test results also indicated that servo transparency was impossible to encounter if the collective was less than 50 percent and also indicated that the flight condition was difficult to enter at airspeeds less than 100 knots. All servo transparency entries and sustainment were accomplished with intentionally high G forces, at high gross weights, and at high entry speeds and were difficult to sustain. All servo transparency conditions during these flight tests ceased when the collective was reduced.
In addition, a Safety Board investigator who is also a rated helicopter pilot conducted several test flights in an AS350 helicopter with Eurocopter’s chief instructor pilot at the company’s Grand Prairie, Texas, facility in December 2003. A Safety Board study of servo transparency data collected from Eurocopter flight tests, SBs, the FAA and Safety Board staff flights indicated that the onset of the servo transparency condition can be predicted based on the helicopter’s weight, altitude, and maneuver loading. The study indicated that, unless the entry maneuver or atmospheric condition inducing the servo transparency condition is violent, pilots would receive a warning, via cyclic control vibration, that servo transparency onset was occurring and that properly trained pilots can exit or avoid this flight condition by proper manipulation of the flight controls.
In addition, flight test data indicated that, when the collective flight control position was at or below the 50 percent raised position, servo transparency did not occur regardless of the helicopter’s maneuvering load, weight, or altitude. Servo transparency onset was possible when the helicopter was subjected to high maneuvering loads at high forward airspeeds during operations at high gross weights and high pressure altitudes but ceased immediately after the pilot reduced the flight control forces, especially the collective, which reduces total maneuvering loads.

[[email protected]]

It's the same bull that Aerospatiale (as they were then) spewed about the Gazelle, they claimed for a while that it was a design feature.

It sounds like they still can't engineer a hydraulic system that can cope with the aerodynamic backloads on their helicopters - a bit like Bell blaming LTE for a TR that was too small for the job.

[SASless]

Pilots and operators may misunderstand this phenomenon.


So nice of the FAA to grant this dispensation!:uhoh:

[victor papa]

Seeing that we are on jack stall immediately associated with an insinuated inferior hydraulic system on the AS350.:ugh: It seems there is about 3900 AS350's flying in the world. The total amount of flight hours is in the 20 millions somewhere if I remember right. How many jack stalls were recorded so far? How many RBS's were recorded so far in the 350? For that matter how many hydraulic failures were recorded?

I do not think the AS350 and it's hydraulic system has much impact on the original interesting subject. The LTE effectiveness phenomena maybe because it is the tail rotors version of RBS. The tail rotor remains an interesting point when you apply all the aerod discussed on main rotors. Maybe we forget sometimes the little rotor at the back.

[Bootneck]

During gazelle conversion we were introduced to the effect.
60* nose down, 60* left wing down and pull the cyclic back sharply, whack! instant level attitude and climb. :ooh:
Next, 30* and 30* to the right, do the same again, with a degree of fear mixed in, whack, onto your right side. :uhoh:

I only had an incipient case once and managed to get out of trouble with a gentle chatter from the jacks as we bottomed out of a dive. (Had to do it my mate was sailing out of Belfast in a sweeper) :)

An Army pilot lost a gazelle in NI as he turned hard right at low level and hauled it around, airspeed being a contributory factor he entered jack stall and it ploughed in. I believe a Colonel of an infantry regt was killed.

[NickLappos]

That service bulletin was obviously written by the manufacturer, who has a vested interest in making sure that jack stall is deemed a universal issue for all helicopters, for obvious, legal, reasons. Otherwise, those who were harmed as a result of this design shortfall would have recourse for damages.

The statement in the service bulletin that says "You should understand that servo transparency is a natural phenomenon for any flyable helicopter" is patently untrue. I have significant experience testing several helicopters to be sure that they cannot and do not experience any control feedback where the controls lock up, become "transparent" or stall because of maneuvering. In fact, NO military helicopter is allowed to have this problem, it is specifically tested for in the structural flight testing. I would enjoy seeing the data that the FAA used to back up this erroneous assertion.

[topendtorque]

a bit like Bell blaming LTE for a TR that was too small for the job.

That is an excellent way to describe it. I’ll remember it when next I am confronted with an LTE “expert”


The Bell 47 G5, when fitted with a 'Texas' No bar Kit, would exhibit feedback forces on the lateral Power cyl when one rolled and turned fairly quickly to starboard. The thing is the lightweight Rotor system was much more manoeuvrable with the No Bar kit and a careless quick turn right would result in a high pucker factor when one decided – fairly quickly I might add- to come right side up.



It was no big deal really, just like flying with hydraulics off but loaded up also, it was however; ONCE ENCOUNTERTED – ALWAYS RESPECTED.

It was a situation that was always amplified with worn power cyl trunnion bushes. I practised hyd-off landings with much worn bushes once, with the Texas kit installed, very spooky!

I did have a pilot once though who thought that the feedback was the result of an out of adjustment irreversible valve spring adjustment. Until I heard about it that is. That really did frighten me.

[topendtorque]

I hope that id did not accidentally delete a couple of messages there. apologies if so. what happened was that the server decided that it was busy just as I submitted, and when I rebooted, there were two of mine, one of which I dumped. talk about jack stall?

[Paul Cantrell]

Nick,

What are the characteristics of a hydraulic system that would be susceptible to jack stall? Is it limitation in pressure or volume that the pump can output, or does it have more to do with servo design?

Paul Cantrell

[ron-powell]

I have to say the hydraulics system on the AS350 is the thing I least like about it.

I have encountered numerous instances of feedback in the controls while in mountain turbulence and "frozen" controls with g loading. Lowering the collective solves the problem easily enough.

Can't say as I agree with anyone who calls this some generic phenomenon.

[ShyTorque]

Paul C, I'm no Nick Lappos but I hope my simple QHI's explanation below helps you to understand what happens:

The main rotor hydraulic control system is there to reduce the feedback forces to the pilot's controls, essentially similar to power steering on a car. The feedback forces originate from effort required to drive the pitch angles of the blades to where the pilot wants them to be and hold them there against variable aerodynamic forces.

A small helicopter is controllable by the pilot without hydraulic assistance due to the small feedback forces involved. A larger aircraft less so, in some cases there is no manual reversion due to the design; instead there are two or more hydraulic systems, giving a failure backup.

As feedback forces get higher, at extremes of speed / and or manoeuvre, the hydraulic system comes under greater strain. This could, subject to design, overstress hydraulic components such as the pump drive, or the rubber seals in the servos, causing complete failure.

To ensure the system cannot be damaged by an overstress, the hydraulic system of a Gazelle has a Pressure Regulating Valve set at 40 bar. The AS 355 has a PRV set at 35 bar. If the system pressure reaches these figures, the valve will open, allowing hydraulic fluid to bleed off back to the reservoir. Whilst this is happening, the jack will effectively stop moving because it cannot overcome the feedback force working against it.

This is what the British military have traditionally called "jack stall". The French recently prefer the term "servo transparency".

As far as the effect on the aircraft is concerned, it will go wherever the aerodynamic forces take it. The pilot can reduce the feedback forces by reducing the severity of the manoeuvre, usually by reducing collective pitch.

If he has allowed the aircraft into a tight corner (literally and figuratively speaking), he may not have that option.

[Paul Cantrell]

ShyTorque,

Thanks for the answer. I must admit it's much simpler than I was thinking - I was assuming it was something like the servos being rate limited or something, but it makes sense that the system needs a pressure relief. I'm just surprised that the system is built without adequate margin for aggressive maneuvering. I guess like any aircraft, you need to watch weight and not over-design. Still, it sounds like it's pretty easy to induce!

Thanks again...

Paul

[starflex3]

What other helicopters can get jack stall or hydraulic transparency apart from AS350, AS355, AS365 and Gazelle.
Is this just a Eurocopter/Aérospatiale thing.
The Gazelle does not have a starflex head like the AS350 or AS365.
??????